Pulmonary arterial hypertension (WHO Group I) is a rare disorder associated with poor survival. Endothelial dysfunction resulting from 1) genetic susceptibility, and 2) a triggering stimulus that initiates pulmonary vascular injury, the two-hit hypothesis, appears to play a central role both in the pathogenesis and progression of PAH. Inflammation appears to drive this dysfunctional endothelial phenotype, propagating cycles of injury and repair in genetically susceptible patients with IPAH and DaPAH. However, despite mounting evidence of vascular inflammation in patients with PAH, detailed phenotypic studies are lacking on the temporal evolution of this process and its contribution to right ventricular (RV) and pulmonary vascular remodeling. In this protocol subjects are thoroughly characterized by echocardiogram, 6-minute walk, cardiopulmonary stress testing, pulmonary function testing, cardiac CT, advanced cardiac MRI techniques, gene expression profiling, and serum biomarkers. We hypothesize that a detailed characterization of these subjects in conjunction with a more rigorous profile of the temporal evolution of vascular inflammation in PAH and its impact on RV and pulmonary vascular function will add prognostic value to traditional measures of disease severity and suggest novel therapeutic targets for future research. The protocol was initially approved by the NHLBI IRB in October 2012. Subsequently IRB approval was sought and obtained at collaborating sites and various amendments were brought into alignment at all institutions by summer 2013. In 2013 -15 we established at the NIH a clinical research PAH program consisting of a multidisciplinary medical staff with expertise in cardiology, pulmonary, and critical care as well as a regulatory and research nursing support staff with extensive experience in assuring compliance with clinical protocols while maintaining the highest standards of clinical care. In 2015-16 reporting period, in collaboration with the Center of Human Immunology at the NIH, we conducted a proteomic profiling study in order to determine the effects of spironolactone on circulating mediators relevant to PAH. Fluorescent bead-based immunoassay (Bio-Plex) was used to measure cytokines in healthy controls (N=24) and PAH subjects (N=55) divided into 3 diagnostic groups (IPAH/Hereditary-PAH, n=26; Connective Tissue Disease Associated-PAH, n=19; and Other n=10 Congenital Heart Disease Associated-PAH, n=7; drug-induced PAH, n=1; hereditary hemorrhagic telangiectasia-associated PAH, n=1; and porto-pulmonary PAH, n=1). Multiple linear regression models were used to adjust for PAH clinical characteristics: Age, gender, race, NYHA/WHO classification (I/II versus III/IV), 6-minute walk distance (6MWD), prostanoid infusion, current use of 1 PAH-specific therapy, and spironolactone. Circulating cytokine levels were elevated across a diverse sample of PAH patients compared to healthy controls. Functional classification and 6MWD modestly tracked with a subset of the tested cytokines. Current therapies did not appear to influence this inflammatory signature, but confounding clinical comorbidities and the relatively small number of subjects may have hindered our ability to detect PAH-treatment related differences. An abstract from this work entitled Serum Cytokine Profiling in PAH was presented at the 2016 International Society for Heart and Lung Transplantation 36th Annual Meeting and Scientific Sessions (JHLT 35(4S):S360, 2016). During the 2016-17 reporting period we entered into a collaboration with the Center for Human Immunology (CHI). We provide CHI with samples that they process for markers of inflammation, and whole blood gene expression studies in order to expand our ability to immunophenotype subjects with PAH. We also entered into a collaboration with the Laboratory of Transplantation Genomics (LTG) of the NHLBI to do exploratory studies using cell-free DNA based tools to evaluate the pathogenesis of and injury related to PAH. We hypothesize that systemic vascular inflammation may exist in PAH and this might manifest itself as increased coronary plaque burden. During the 2017-18 reporting period we have been performing post imaging processing to obtain 3D global reconstructed projections of the heart from our cardiac CT imaging to assess coronary plaque burden in PAH compared to healthy subjects. We also continue to do post image processing of our serial assessments of cardiac chamber volumes and masses by MRI and echocardiography to compare to our serum biomarkers, cytokine, genomic profiles, and functional assessments. In addition, we have completed preliminary studies assessing our ability to measure cell-free DNA (cfDNA) in PAH patients as a potential marker of cardiovascular dysfunction. During the 2018-19 reporting period we continued to assess coronary plaque burden by CT, cardiac chamber volumes/mass/function by MRI and echo, pulmonary function parameters, exercise capacity, coagulation status, serum biomarkers, cytokine and genomic profiles. In addition, plasma cfDNA levels was quantified and compared between disease severity groups. The protocol is open for enrollment. Referrals are currently being received from multiple sites. During the current reporting period 7 subjects were enrolled (3 healthy volunteers and 4 patients). Over the life of the protocol (as of September 2019) 275 charts of potential subjects have been reviewed for this study and a total of 62 subjects have been enrolled (52 patients and 10 matched controls). During the 2018-19 reporting period the following abstracts was presented. The 2019 American College of Cardiology 68thAnnual Scientific Session: Plasma Cell-free DNA as a Novel Marker of Disease Severity in Pulmonary Arterial Hypertension (J Am Coll Cardiol, 73(9, S1): S1897, 2019); The 2019 American Thoracic Society International Conference: Coronary Artery Plaque Burden in Patients with Pulmonary Arterial Hypertension(Am J Respir Crit Care Med, 199:A6795, 2019). The following abstract will be presented at the American Heart Association Scientific Sessions, November 2019: Lu M, Blaine K, Cullinane A, Hall C, Dulau-Florea A, Sun J, Chenwi H, Graninger G, Harper B, Elinoff JM, and Solomon MA. Pulmonary Arterial Hypertension Patients Display Normal Kinetics of Clot Formation.